It is known that video workstations can be outfitted with video capture hardware in an inexpensive manner. Combined with audio hardware that is already available, such systems are capable of video conferencing.
In order to render such techniques practical, however, it is necessary to compress the video signals by a large factor, such as 20:1, and it is necessary to effect such compression in real time. It is of course preferable that compression of the signals at a transmitting station, and decompression of the signals at a receiving station, be effected without the necessity of additional equipment.
Video compression techniques generally use two strategies to reduce the amount of information that is needed to represent a moving image. The first of these techniques takes advantage of the similarities between consecutive frames of an image, and effects the transmission only of signals corresponding to portions of the picture that have changed. The second technique effects the encoding of signals corresponding to portions of the image that have changed, and the transmission only of the encoded pixels, in order to minimize the time required for the transmission of data.
It is advantageous in many cases to compress video signals by "lossy" compression techniques. Such techniques enable the attainment of an increased compression ratio. Such lossy compression techniques are disclosed, for example, in the article "The JPEG Still Picture Compression Standard", Gregory K. Wallace et al, submitted in December 1991 for publication in the IEEE Transactions on Consumer Electronics, as well as a paper by the same name, and of the same authors, that appeared in the April 1991 issue of Communications of the ACM.
Since some aspects of the preferred compression technique in accordance with the present invention differ, in part, from the JPEG (Joint Photographics Experts Group) standard discussed in the above publications, a discussion of the compression techniques in accordance with the invention will be presented only in following disclosure of the invention.
It will be understood, however, that, generically, the term "lossy" compression refers to compression techniques wherein, although data may be lost in the process of compression, an increased compression ratio is obtained.